This invention relates to amplifiers, to audio amplifiers, and particularly to a single-ended push-pull amplifier circuit suitable for use at the final stage of multistage audio amplifiers.
Complementary single-ended push-pull amplifiers (FIGS. 1 and 2) have been known and used extensively for the final-stage power amplification of audio signals. Japanese Unexamined Patent Publication No. 55-35520 suggests improvements relating to amplifier circuits of this broad category.
The typical push-pull amplifier of the kind under consideration has two amplifying transistors, one connected between a positive supply terminal and an output terminal, and the other between a negative supply terminal and the output terminal. The output terminal is grounded via a loudspeaker system. Two other transistors are employed for driving the amplifying transistors. The drive transistors have their base current controlled according to the incoming audio signal, thereby controlling current flow through the amplifying transistors. Thus is the loudspeaker system supplied with the current amplified according to the magnitude of the input signal.
For operation as a class A amplifier, the biasing circuits incorporated into the amplifier circuit are so preset that idling current flows through the transistors at all times, even when the signal input is zero. Unless kept under constant control, however, the idling current could cause the overheating and eventual breakdown of the transistors. The idling current should not be so great in magnitude as to lead to the transistor overheating but so low as to cause switching distortion, as in class B amplifiers.
Various suggestions have been made, and some put into practice, for precluding the thermal breakdown of the transistors. Some such known suggestions, bearing particular pertinence to the instant invention, are unsatisfactory by reason of too delicate preadjustment of some circuit elements that must be made at the cost of considerable time and labor to provide idling current of a desired level. Another objection is the prolonged period of time required by the prior art amplifiers of the type in question for the idling current to rise to the required level after the amplifier is switched on.
The present invention seeks to make totally unnecessary the time-consuming preadjustment conventionally needed by the class A amplifiers of the type defined to assure the flow of constant-level idling current in the face of temperature variations of the relevant circuit elements such as transistors.
The invention also seeks to provide for the stabilization of the idling current in the amplifier circuit just after it is switched on, virtually eliminating the warm-up period heretofore required for the idling current to gain its steady-state magnitude.
Briefly, the present invention may be summarized as a single-ended push-pull amplifier circuit suitable for power amplification of audio signals, among other applications. Included are first current control means (e.g. two transistors in Darlington connection) connected between a first supply terminal and an output terminal for controlling current flow therebetween, and second current control means (e.g. two other transistors in Darlington connection) connected between a second supply terminal and the output terminal for controlling current flow therebetween. A biasing circuit is provided according to the invention which comprises a serial connection of a first and a second resistor, the latter being connected to a control terminal of either of the first and the second current control means, and a first and a second bias transistor. The first bias transistor is connected between a control terminal of the other of the first and the second current control means and the first resistor, and has a control terminal connected to said other of the first and the second current control means. The second bias transistor is connected between the control terminals of the first and the second current control means and has a control terminal connected to a junction between the first and the second resistor.
Such being the improved construction of the amplifier circuit, particularly the biasing circuit, according to the invention, the current through the bias transistors increases in magnitude with the current through the current control means, resulting in negative feedback of the control voltages for the current control means. The current through the current control means is therefore stabilized without waiting for the warming of the bias transistors.
Another pronounced advantage of the improved biasing circuit is that the magnitude of the idling current becomes independent of temperatures merely as the two resistors connected in series with the first bias transistor are made equal in value. The resistors can be fixed at the same value. No readjustment whatever prior to shipment, and of course in use, is necessary for provision of an idling current of desired magnitude.
In an alternative embodiment of the invention a diode is connected in series with the resistors for additional temperature compensation. Various other embodiments will be disclosed to show various forms the amplifier circuit according to the invention can take in practice.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings showing the preferred embodiments of the invention.